Phil is correct as usual.
Without knowing any of your particulars. (Ever notice that no one ever gives any information yet wants help). I would say stick to a full body three times per week workout. Keep your sets to 25 or less per session. Keep your sessions to 60 min or less. Keep your reps in the 6 to 12 range. Keep your reps to one or two short of failure.
Take a break of 5 to 7 days after an eight week program.
Eat six times per day. Take in one gram of protein for every pound of body weight. Sleep 8 to 10 hours per day.
Oh and you will probably need to stock up on Grow! protein powder as this will be one or two of your six meals per day. Easy and tastes great!
I have gained up to 12 pounds of solid muscle in one years time doing this (while in my late 30's). Some do better, others not as well.
It's all pretty simple you just have to discipline yourself to do it!
If there is ever anything that I can help you with don't hesitate to PM me.